For light emitting diodes (LEDs) for use in lighting applications, it is desirable to provide essentially white light having a color temperature approximately comparable to that produced by incandescent lighting.
White light from LEDs is commonly provided by using a pn-diode emitting blue light, having a wavelength around 450 nm, where part of the blue light is converted to longer wavelengths using one or more wavelength converting materials arranged on top of or in the vicinity of the diode. Together with the blue light which is not absorbed, light having a reasonably broadband spectrum which is perceived as white light can be obtained.
Currently, in most commercial applications, the wavelength converting material is applied directly on the LED. Furthermore, the wavelength converting material should be scattering in order to obtain a low variation in color over angle. This means that blue light will also be scattered back into the diode which leads to absorption losses in the LED. Moreover, the active component of the wavelength converting material, commonly phosphor, is an isotropic emitter, meaning that the same amount of wavelength converted light is emitted in all directions, leading to further losses as only a portion of the light reaches the surface of the light emitting device.
The problem of reducing losses has for example been addressed by using a phosphor which is less scattering to reduce the amount of blue light which is backscattered and absorbed by the diode. However, the isotropic emission from the phosphor remains.
The amount of light leaving the light emitting device may also be increased by introducing a photonic band gap material in which the emission direction can be modified. However, to be able to control the emission direction, a photonic band gap material needs to be made from materials having a high refractive index contrast, high aspect ratio holes or pillars must be patterned and formed, the size control is very strict and the material must be luminescent which will incur scattering losses. Furthermore, a photonic band gap material is only really effective in the plane perpendicular to the surface of the material, i.e. in a direction parallel to the holes or pillars.
Accordingly, the suggested approaches for increasing the emission efficiency of a light emitting device suffer from inherent drawbacks which are hard to overcome.